1. Technical Field
The present disclosure relates to Power Amplifiers (PAs) used in Radio Frequency (RF) transmitters.
2. Background Information
FIG. 1 (Prior Art) is a diagram of a typical cellular telephone handset 1. Handset 1 includes a Digital Baseband Processor Integrated Circuit (DBPIC) 2, a Radio Frequency (RF) transceiver Integrated Circuit (RFIC) 3, Power Management Integrated Circuit (PMIC) and support circuitry 4, a Switching Mode Power Supply (SMPS) 5, an external Power Amplifier integrated circuit 6 (PA), a duplexer 7, an antenna switch 8, an antenna 9, and a set of matching networks 10-12. RFIC 3 includes a transmitter portion 13, a receiver portion 14, a power detector portion 15, a set of control and interface registers 16-18, and a serial bus interface 19. DBPIC 2 includes a Digital-to-Analog Converter DAC 20 for supplying an analog signal to the transmitter portion 13, an Analog-to-Digital Converter (ADC) 21 for digitizing an analog signal received from the receiver portion 14, and a serial bus interface 22 for communicating via serial bus 23 with RFIC 3. In addition, DBPIC 2 includes a local bus mechanism 24, a processor 25, and an amount of memory 26.
External PA integrated circuit 6 is used to amplify an RF input signal received from the transmitter portion of RFIC 3, and to deliver an amplified version of the RF signal at an increased power level indirectly through circuitry 12, 7, and 8 to antenna 9. The handset can output an RF signal from the antenna at one of a set of “transmitter output power settings.” A Base Station (BS) can send a request to the handset. The request instructs the handset either to increase its “TX output power setting” one increment or to decrease its “TX output power setting” one increment. There are two components to achieving such a TX output power. One is the amplitude of the RF signal as supplied to the input of the PA. The other is the power gain of the PA and the subsequent loss of the amplified RF signal through the matching network 12, duplexer 7, and switch 8. When the BS sends a request that the handset output at its maximum TX output power setting, then DBPIC 2 supplies an appropriate RF signal amplitude to the PA 6 and sets the gain of the PA using a two-bit COARSE GAIN CONTROL signal on conductors 27. The result is that the handset outputs the correct amount of TX output power to antenna 9.
FIG. 2 (Prior Art) is a diagram that shows a characteristic of a TX output signal as transmitted from the handset. The TX output signal of interest is the 1.2288 MHz wide portion 28A indicated in the diagram. When this RF signal is amplified by PA 6 and is transmitted from antenna 9, nonlinearities in the transmitter including PA nonlinearities cause undesired “Out-Of-Band” (OOB) power (“shoulders” or “spectral regrowth”) to be transmitted along with the desired signal of interest. When a handset is made, it is required that the handset meet certain requirements referred to here as Minimum Performance Standards (MPS) requirements. These requirements include a first requirement that linearity of the transmitter be of a minimum linearity (that OOB power is below a certain power as compared to the power of the desired signal of interest). In the example of FIG. 2, there is a 30 kHz range 28B that is centered at a frequency that is 885 kHz below the 0.880 GHz center frequency of the desired signal. Similarly, there is a 30 kHz range 28C that is centered at a frequency that is 885 kHz above the 0.880 GHz center frequency of the desired signal. It is required that the OOB power in each of these two 30 kHz ranges 28B and 28C be −42 dB or less as compared to the power of the desired signal in band 28A. The MPS compliance requirements also include a second requirement that the handset output a specified amount of output power when it is set to output at its maximum “TX output power setting”.
Many PA integrated circuits today have two supply voltage (VCC) terminals. VCC1 terminal 30 in FIG. 1 is a first such supply terminal. VCC2 terminal 31 in FIG. 1 is a second such supply terminal. In the configuration set forth in FIG. 1, the two terminals 30 and 31 are supplied with the same supply voltage VPH_PWR in what is referred to here as the “battery direct” configuration. Two VCC terminals are provided rather than one for reasons related to reducing interference and noise interactions between different parts of the circuitry within the PA integrated circuit.
FIG. 3 (Prior Art) is a simplified diagram that illustrates the battery direct configuration circuit of FIG. 1. As the amplitude of the RF output signal increases and decreases, the supply voltage supplied onto the VCC1 terminal 30 and the VCC2 terminal 31 of PA 6 remains fixed.
FIG. 4 (Prior Art) is a simplified waveform diagram that illustrates operation in the battery direct configuration. As indicated by waveform 32, the power of the TX output signal increases and decreases over time. The supply voltage supplied onto the VCC1 terminal 30 and onto the VCC2 terminal 31, however, remains fixed. Waveform 33 represents this fixed and regulated DC voltage VBATT.
FIG. 5 (Prior Art) is a simplified diagram that shows how changing VCC1 and VCC2 in this battery direct configuration affects TX output power and battery current drawn by the PA.
A PA integrated circuit typically has a characteristic in that if the PA is outputting a signal of a certain output power, then the supply voltage VCC2 supplied to the PA can often be reduced from its supply voltage maximum without substantially affecting PA performance (the supply voltage supplied onto VCC1 terminal 30 is held substantially constant). The current drawn by the PA remains fairly constant despite the falling supply voltage VCC2. A handset using the PA in this way will remain in “compliance” with the MPS requirements. But as the supply voltage VCC2 is decreased further and further, there comes a point where PA performance is significantly affected and eventually the handset fails to meet the MPS requirements. This point is in part determined by the amplitude of the signal being output by the PA. The voltage difference between the maximum of the amplitude of the output signal and the maximum supply voltage VCC2, multiplied by the supply current (the supply current is fairly constant for these high supply voltages), is power. This power is largely wasted in the PA in the form of heat. To conserve power in the handset, it would be desirable to be able to power the PA with the minimum supply voltage VCC2 so as to save power while maintaining MPS compliance.
FIG. 6 (Prior Art) is a diagram that shows how reducing supply voltage VCC2 has a minor impact on PA linearity 34 as the supply voltage VCC2 is reduced from 3.6 volts to about 2.6 volts, but as the supply voltage VCC2 is reduced further below 2.6 volts the PA linearity 34 decreases faster. PA TX output power 35 remains relatively constant over the entire 3.6 volt to 2.1 volt VCC2 supply voltage range.
Unfortunately, a duplexer does not have the same performance as the PA across the entire frequency range of operation (the BC0 band that has a bandwidth of 25 MHz is used in the following description). The drooping line 36 in FIG. 7 (Prior Art) represents PA gain in an individual handset. The duplexer has more insertion loss at the low end of the 25 MHz band and at the high end of the 25 MHz band as compared to the insertion loss in the middle of the band. The gain of the PA is therefore made to be larger at these ends of the band so that overall handset gain to the antenna is relatively constant across the band, regardless of where in the band the 1.2288 MHz RF signal to be transmitted is located. The 1.2288 MHz RF signal is made to move around in the 25 MHz band as the handset operates. There are discrete “frequency channels”, whose center frequencies are separated from one another by 30 kHz. The 1.2288 MHz signal is transmitted in one of these frequency channels. 1.2288 MHz is much wider than 30 kHz, so two RF signals in adjacent frequency channels would overlap each other to some extent.
In addition to variations in the performance of one duplexer as a function of frequency, there are also variations in the components of the handset from unit to unit. Handsets are made in a factory by the millions, and it is too time consuming (and therefore too costly) to calibrate each handset unit on the factory line to test its linearity and output power characteristics for a certain VCC2 to assure compliance at the handset's maximum TX output power setting. As a result, all the handsets are made to use a higher VCC2 supply voltage setting than would ordinarily be necessary to achieve compliance in an individual handset just so that a few handset units that function relatively poorly will also be compliant.
In FIG. 7 (Prior Art), the vertical separation between the stair step VCC2 line 37 and the stair step VCC2 line 38 represents the extra margin of VCC2 that is provided. Note that in this possible solution, the VCC2 supply voltage is different depending on where in the 25 MHz band the 1.2288 MHz RF signal of interest lies. The VCC2 level of a particular handset is not typically reduced to the point of the handset unit just barely meeting linearity and output power requirements. The PA of the typical handset unit is supplied with a VCC2 supply voltage that is significantly higher than required by the typical unit.
FIG. 8 (Prior Art) is a circuit diagram of a second item of prior art referred to here as “envelope tracking” In addition to the circuitry of FIG. 1, the handset of FIG. 8 includes a high-speed switching mode power supply 39, a terminal 40, a special receiver portion 42, another ADC 43, and a conductor 44. Power supply 39 has the capability of changing VCC2 (not VCC1) very rapidly as the amplitude of the RF signal changes. Supply voltage VCC2 is not set at a fixed level for a given frequency channel and left at that level, where the VCC2 level of each frequency channel may be different. Rather, the VCC2 level is rapidly changed during handset transmission in one frequency channel, and this rapid variation is a function of power of the TX output signal. A digital control signal or signals CONTROL supplied via conductor or conductors 44 control the power supply 39 to vary the supply voltage VCC2 appropriately. Reference numeral 45 represents a splitter. Reference numeral 46 represents a 20 dB coupler.
FIG. 9 (Prior Art) is a simplified diagram of the envelope tracking configuration of FIG. 8. Supply voltage VCC1 supplied to onto VCC1 terminal 30 of the PA 6 is constant. The supply voltage VCC2 supplied onto VCC2 terminal 31, however, is made to vary as a function of the power of the I and Q components of the signal being amplified by the PA.
FIG. 10 (Prior Art) is a simplified diagram that illustrates operation of the envelope tracking configuration circuit of FIG. 8 and FIG. 9. Dark line 47 represents the supply voltage VCC2 supplied onto VCC2 terminal 31 of PA 6. Note that the amplitude of supply voltage VCC2 tracks the envelope of the TX output signal 48.
To realize envelope tracking, the special receiver 42 is provided to monitor the amplitude of the RF output signal. Coupler 46 supplies some of the RF output signal as output from the power amplifier 6 back to the special receiver. The output of the special receiver 42 is digitized and is then processed in DBPIC 2 to make a determination of the power amplitude of the time varying TX output signal. The resulting power determination is then used to adjust the level of supply voltage VCC2 output by Switching Mode Power Supply 39 to PA 6. DBPIC 2 controls the level of supply voltage VCC2 via control signal CONTROL on conductor or conductors 44. As illustrated in FIG. 10, the supply voltage VCC2 is not fixed for one combination of frequency channel setting and TX output power setting, but rather the supply voltage VCC2 closely tracks the magnitude of the amplitude of the TX output signal as illustrated in FIG. 10.
Although envelope tracking reduces power consumption of the transmitter circuitry, it can be difficult to adjust the VCC2 power supply voltage fast enough and accurately enough in certain situations. If not done correctly, there can be substantial distortion of the signal of interest, and receive band noise can be generated.